It is a special type of alkaline mineral salt with a distinctive/sulphurous flavour. The sulphur content gives it an eggy taste which is useful for flavouring egg less dishes like tofu scramble or quiche.
Black salt or Kala Namak is not black, but more a reddish-grey, because of the presence of trace minerals and iron. It is not interchangeable with pink Himalayan salt because of its distinctive flavour.
100% Black Himalayan Salt